A harp is a chordophone instrument provided in general with 47 strings, for most of which it is possible to carry out the sharp and flat changes by operating pedals or special levers.
Concert harps are usually manufactured using traditional methods, resorting primarily to the use of wood. The criteria for the design and manufacture are defined in a rather empirical way, and therefore the features of the manufactured instruments often suffer from poor reproducibility and reliability.
The “heart” of a harp is represented by the sound chest: this chest is in effect the main bearing structure, and constitutes the source of the vibroacoustics and timbre of the instrument.
The main parts of the sound chest of a harp are the sound board and the associated hollow body or shell, which operate together with each other but with respective different roles.
The sound board primarily has to accomplish two seemingly antithetic tasks: on the one hand, it has to accomplish a structural function for providing stable and secure opposition to the pull applied by the strings, and on the other hand it has to have high elasticity for effective transformation of the vibration of the strings into sound output.
The hollow body or shell of the sound chest comprises a bearing structure, the main elements of which are represented by the battens (backboard and two side planks), which extend between the chest-bottom and the upper block. This structure is transversely stiffened by a plurality of (generally four) longitudinally staggered, transverse members or bridges.
A closing shell manufactured typically with a multi-ply sheet of maple wood is applied to this bearing structure with a thickness of about 5 mm. This shell is bent and glued to the chest-bottom, to the upper block, to the side planks and to the backboard by a vacuum press, with an associated heating system which can make the bonding sufficiently quick.
During operation, the bent multi-ply shell is subjected to stresses which tend to cause it to deform inwards in an implosive manner (so-called “buckling”), which is extremely undesirable. Indeed, this deformation not only changes the shape of the chest in an aesthetically unappealing manner, but is also structurally unsuitable, since it makes the chest system excessively elastic, creating serious problems in terms of the stability of the sound board and the tuning In order to limit these phenomena and increase the rigidity of the shell without increasing the weight excessively, the prior art provides for encircling transverse members or small bands to be glued to the internal surface of the shell, intended to limit the deformation of the bent multi-ply wood with which the shell is formed.
In order to further increase the bending strength of the shell, the shape thereof essentially as half a truncated cone is “swelled up” like a barrel, with a rather marked bending camber by the backboard and the side planks. This “swelling up” like a barrel creates considerable problems in manufacture, in particular in the step for bending the multi-ply maple wood with which the shell is formed: it is possible indeed for there to be instances of internal strain created in the plies, likely to produce delamination and also “voids”, while considerable folds or wrinkles can form on the external surface of the shell on account of compression of the outer plies. The elimination of these design defects requires an important step for manually finishing the external surface of the shell, by scraping and sanding, in order to make the shell aesthetically acceptable and return the shape to the established dimensions.
Depending on the nature and the extent of the defects created during the moulding of the shell, the external finishing involves the removal of plies of differing thickness, and therefore the finished shell does not have a constant thickness in all the parts thereof. This alters the overall rigidity profile of the shell and produces a dispersion in the characteristics of the bending behaviour of the chest, as well as between one chest and another.
As regards the bearing structure of the sound chest, the prior art provides for the connection between the bridges, the backboard and the internal planks to be realized by the juxtaposition of the planar faces thereof, between which an adhesive is interposed. This way of implementing the connections is unsatisfactory, since the various components are assembled in temporally successive stages, with manual repair interventions which cause dispersions in the characteristics of the manufactured product.